Pickling of ferrous metals



Patented Nov. 26, 1946 UNITED STATES PATENT OFFICE PICKLING OF FERROUS METALS No Drawing. Application May 16, 1944, Serial No. 535,880

8 Claims. (Cl. 252-151) The present invention pertains to the pickling of metals for the removal of rust, scale and other undesirable impurities. While it is applicable to the pickling of various metals, it was conceived and perfected in research directed to discovery of a suitable inhibitor for use in baths employed in pickling of iron and steel, and will be described speciflca ly in reference to that problem.

In the processing of steels, a hard scale of metallic oxides, predominantly iron oxides, is formed on the surface. This scale is injurious to the tools and dies employed in finishing and is accordingly generally removed by mechanical and chemical means. The scale may be partially removed by bending of the steel in a scale breaker, or completely removed by sand blasting or tumbling with shot. In spite of the availability of these physical methods, however, most descaling operations are accomplished by treatment with pickling acids, of which the commonly used acid is sulfuric acid. The action of the acid in removing the scale is accomplished by dissolving the scale and also by attacking the steel in exposed areas and working back under the edges of the scale. The hydrogen evolved when the acid attacks the steel assists in dislodging and removing the scale. The action of the acid is rapid, both upon the scale and upon the metal itself, with the result that the process can become quite expensive both because of consumption of acid and because of loss of steel, if the steel is allowed to remain in contact with the acid for too long a time. Contact of the acid with the steel for a period even as short as ten minutes after all of the scale has been removed can result in substantial loss of steel. In order to reduce this loss, it is common practice to add to the acid a small proportion of a substance capable of acting as a pickling inhibitor to slow down the action of the acid on the steel. A necessary incident to the use of an inhibitor, however, is that it entails a retardation of the pickling action. The best that can be accomplished is to choose the type and amount of pickling inhibitor and time of treatment in such a manner as to effect the best economic compromise between the alternative evils of excessive loss of the metal under treatment and excessive consumption of time in the pickling operation.

The attack of the acid upon the metal does not always proceed uniformly over the entire surface. This is particularly true when the steel has been scale-broken or when part of the scale has been broken oil during handling. A phenomenon called diilerential pickling frequently occurs under these conditions, and is characterized by the occurrence of pits and blisters on the metal. This is not serious in the pickling of forgings and billets, but when finished steel is pickled in the form of its approximate final dimensions, the imperfections show up in the final product. An important feature of the present invention consists in the fact that it provides an unusually effective inhibitor for minimizing, and in most cases entirely preventing, this phenomenon of differential pickling.

The invention is based upon the discovery that alkylene thioglycols, i. e., compounds which may 'be regarded as aliphatic hydrocarbons substituted both by sulfhydryl and hydroxy radicals, act as excellent corrosion inhibitors for use in pickling baths, and serve to prevent the objectionable diflerential pickling, as discussed above. In the more specific form of the invention, these thioglycols are combined with compounds which are closely related to them, to wit, the his hydroxy alkyl thioethers. While the hi0 1 1s and he correspondin h use indemndently as pickling inhibitors, experimentation with use of these compounds separately, and comparison of the results with use of the two types of compound together, have demonstrated that the compounds have a synergistic efiect upon one another in the inhibiting of corrosion; i. e., the use of a given weight of a mixture of the two types has a much greater inhibiting effect than does the use of the same weight of either of the types individually. When a mixture of ethylene thioglycol with the corresponding bis beta hydroxy ethyl thioether is used, for example, the inhibiting effect is greater than can be attained by use of an amount of either the thioglycol or thioether equivalent to the weight of the mixture used. The same general comment applies to use of propylene thioglycol and corresponding hydroxy propyl thioether and to other analogues and homologues of these compounds.

The economic importance of this synergistic promoting eflect becomes more evident when we consider that the hydroxy alkyl thioethers are obtained in the same reaction by which the thioglycols are produced. Thus, when one molecule of hydrogen sulfide is reacted with one molecule of ethylene oxide, the resultant of the reaction is ethylene thioglycol:

1. (C3930 +HQS --b CHr-CH H H Ethylene oxide Ethylene thioglycol When the ethylene thioglycol of the above reaction is condensed with a further molecule of ethylene oxide, the corresponding bis beta hydroxy ethyl thioether is formed:

2. (CHghO CHr-CH; -h CHg-CHg-S-CHrCH:

H H H Ethylene oxide Bis beta hydroxy ethyl thioethar In actual practice, the resultants of both of Equations 1 and 2 are formed in condensation of ethylene oxide with hydrogen sulfide, regardless of the exact proportions of ethylene oxide and hydrogen sulfide used. Under ordinary circumstances, this might be considered a detriment, since a purification operation would be required to obtain the resultant of either of Equations 1 or 2 in substantially pure form. As noted above, however, the resultants of these equations are not required in pure form for practice of the present invention, and an actual advantage is obtained by use of the crude reaction mixtures, containing both types of compounds. While the above explanation is applied with reference to equations illustrating reaction of ethylene oxide with hydrogen sulfide, it is equally applicable to reaction of other alkylene oxides with hydrogen,

sulfide to produce homologous mixtures of the higher thioglycols with the corresponding thioethers, and to similar mixtures which are obtained when these compounds are prepared by the alternative procedure of reacting the correspondin chlorhydrlns with alkali metal hydrosulfides.

While mixtures of alkylene thioglycols and the corresponding hydroxy alkyl thioethers (and these compounds individually) may be successfully used in practice of the invention regardless of the carbon content of the individual alkylene radicals, compounds of this type containing between 2 and 5 carbon atoms in each alkylene radical are preferred for practice of the invention, and the ethylene and propylene compounds have been found to be especially useful. These ethylene and propylene compounds have been used successfully in inhibiting corrosion in the pickling of steel when used in volume concentrations of between 0.037% and 0.094%, based upon the amount of dilute sulfuric acid used in the pickling bath, and upon temperature and other conditions. Thus, excellent results have been obtained in pickling a variety of alloy steels by use of one pint of inhibitor for each 40 gallons of 66 B. sulfuric acid, the acid being diluted with sufficient water to give a concentration of 12% by volume of the 66 B. acid, when temperatures between 150 and 165 F. are used in the pickling operation. At higher temperatures, such as 195 F., it is best to use larger quantities of the inhibitors, and amounts as great as the 0.094% indicated above and even higher can be used to advantage.

The use of the thioglycols and related compounds and mixtures discussed above as pickling inhibitors has produced outstanding results in the prevention of localized attack, variously referred to as burning, blistering and pitting, of a wide variety of steel. With many steels, the pickling time can be materially reduced with these inhibitors as compared to other available inhibitors, without increase in loss of the steel. This is apparently due to the more rapid hydrogen evolution rate permitted by the inhibitors of this invention when compared with similar volume concentrations of well-known marketed inhibitors under identical conditions. Whereas other inhibitors tested in comparison with those of the present invention allowing pitting to co- 4 our in about direct proportion to the amount of steel dissolved, the inhibitors of the present invention protect the steel against uneven attack even at relatively high hydrogen evolution rates.

In addition to the smoothness and brightness of the drawn stock obtained when the pickling inhibitors of the invention are used, and the increased rate of pickling obtained, these inhibitors provide protection for the metal when used in pickling baths of high iron content, and they have additional advantages in that they can be easily incorporated in the dilute acid of the pickling bath to provide an inhibited pickling bath of uniform composition. The ability of the inhibitors to protect the steel in the presence of a relatively high proportion of iron salts makes it possible for more steel to be pickled before the spent acid must be dumped. Most pickling inhibitors are soluble in pickling baths to only a limited extent, and the and %i o glygol and hydroxy thioether I bitors of t e present invention provide important advantages in this regard in that they are miscible with water and dilute acid in all proportions; "Since Examples The following tabulated data were obtained by exposure of cold drawn steel bars to dilute sulfuric acid of 12% strength by volume of 66 B. acid (18.7% by weight) inhibited by one or 2.5 pints (except in the single instance of inhibitor F in run 9) of various inhibitors per 40 gallons of 66 B. acid, which is equivalent to a concentration of 0.037% or 0.094%, respectively, by volume. Cold drawn steel bars were employed in preference to unpickled bars in order to assure a uniform surface for comparison of the several inhibitors. The temperature in each case was either F. or 194 F., most of the tests being run at 194 F., because inhibitors effective at this very high temperature would also be satisfactory at lower temperatures, although the converse would not necessarily be true. At lower temperatures the time of pickling would, of course, be increased. In determining the ability of the inhibitor to maintain its inhibiting function, each bar was replaced with a new bar after exposure for to 3.5 hours. In this manner, identical surfaces were exposed during various stages of the life of the pickling bath. In every case, the thioglycol maintained its inhibiting capacity after repeated uses, whereas some of the other inhibitors rapidly lost their effectiveness upon repeated use. The letters A, B, C, D, E and F refer to well-known currently marketed inhibitors, and the letters TG refer to thioglycol. The data given with respect to the respective runs provide a comparison between the effectiveness of the thioglycol inhibitor and the other inhibitors under equivalent conditions, as discussed above.

Inspection of the table reveals that the thioglycol inhibitor provides less protection against acid attack than some of the other inhibitors with some grades of steel. The surface condition of the steels when thioglycol is used as the inhibitor is, however, excellent in almost every case, and the results as to loss of metal must be considered in conjunction with this factor in interpretation of the data of the table. practice, in cases in which the attack of the ac 6.

upon steel is rapid, the steel would not be exposed to the acid inhibited with thioglyool for as long a time as was done in some of these experiments. By making a proper adjustment of the time factor, the pickling function can be accomplished while maintaining loss of steel at a minimum. An important advantage of the use of thioglycol as compared to any of the other inhibitors 6 4620, 4635, 4640, 6150, 8739 and 52,100 steel were pickled at 160-180" F. One pint of crude thioglycol was employed for every 40 gallons of 60 B. sulfuric acid, The concentration of acid dropped from 12.0% (by volume) to 5.5%, while the iron content of the acid at the end of the run was 0.4 pound per gallon. Under similar conditions, 152,455 pounds of steel of grades x1314, x1335, 3312, 4035, 4130, 4140, 4620, 8739 and 52,100 were pickled using 1.3 pints of inhibitor B for each 40 gallons of 60 B. acid. The acid strength dropped to 6.5% and the iron pickup was 0.5 pound per gallon. These two plant runs demonstrated that the inhibitors were about equal in regard to iron pickup and acid consumption, but

that the quality of the pickled steel is better and the pickling time is shortened by the use of thioglycol and its mixtures with bis beta hydroxy ethyl thioether.

Inhibitor contcentrast 1 Run ion, ea a Loss No. Inhibitor 8. Temp., F. (mg/my) Surface condition of here Be. sulfuric acid 1 3, 140 150 2. 5 Slightly pitted. 1 3,140 150 5. 8 Fitted. 1 3. 140 150 43. 9 Smooth. 1 3. 140 150 66. 4 Ruined.

2. 5 4, 635 194 41. 7 Pitted. 2. 5 4. 635 194 118.0 Sides smooth; ends blistered. 2. 5 4, 635 194 153. 5 Badlyegitted. 2. 5 4, 635 194 1.050. 6 Ruin 2. 5 2, 330 194 8. 9 Smooth. 2. 5 2. 330 194 5. 9 Do. 2. 5 330 194 19 2 Slightly pitted.

2. 5 52, 100 194 2.0 Smooth. 2. 5 52.100 194 7. 4 Slightly pitted. 2. 5 52, 100 194 12. 7 Smooth.

l 2, 330 194 30. 8 Smooth. l 2, 330 194 213. 6 P tting proportional to weight loss. 1 2, 330 194 747. 1 First bar badly pitted; others mined.

1 52. 100 194 4. 3 Slightly pitted. 1 52. 100 194 9. 5 Pitted. 1 52, 100 194 103. 5 Smooth.

1 3, 135 194 128. 3 Slightly pitted. 1 3, 135 194 140. 3 Pitted. 1 135 194 257. 8 Smooth.

2. 5 4, $5 194 6. 2 Pitted. 2. 5 4. 635 194 6. 4 Smooth. 2. 5 4, 635 194 48. 9 D0. 2. 5 4, 635 194 1, 240. 7 Ruined.

2. 5 4, 635 194 5. 0 Smooth. 2 5 4, 635 194 41. 7 Do. 2. 5 4, 63 194 49. 4 One end pitted; sides smooth. 2. 5 4, 615 194 6. 5 Pitted. 5. 0 4, 635 194 72 8 Do.

10.-.. Synthetic 'lG Thiogly col 72% 2. 5 4, 635 194 7. 9 Smooth.

Beta hydroxy ethyl thioether 28%--."

Crude TG 2. 5 4. 635 194 9. 3 Do.

Although the above laboratory data were obtained employing cold drawn steel bars, plant scale tests have demonstrated the effectiveness of thioglyool and mixtures of thioglyool with beta hydroxy ethyl thioether in protecting steel during Picklin Thus, 33,430 pounds of S. A. E. 1035, 4027, $4340 and 4635 steel were pickled with excellent results. The tub contained one pint of thioglycol for each gallons of 66 B. sulfuric acid, the initial concentration of sulfuric acid was 10.5% by volume, and the temperature was held between 140 and 160 F.

In another set of tests designed to determine the acid consumption and iron pickup in comparison with inhibitor B, 162,845 pounds of S. A. E. 21335, 3135, 3140, 3312, 4035, 4140, 24340,

Various modifications are possible within the scope of the invention, and we do not therefore wish to be limited except by the scope of the following claims.

We claim:

1. The process of pickling ferrous metals which comprises subjecting the metal to be pickled to the action of a sulfuric acid pickling bath containing as a corrosion inhibitor an alkylene thioglyool having between 2 and 5 carbon atoms.

2. The process of pickling ferrous metals which comprises subjecting the metal to be pickled to the action of a sulfuric acid pickling bath containing as a corrosion inhibitor ethylene thioglycol.

3. The process of pickling ferrous metals which comprises subjecting the metal to be pickled to the action of a sulfuric acid pickling bath containing as a corrosion inhibitor propylene thio- ILvcol.

4. The process of pickling ferrous metals which comprises subjecting the metal to be pickled to the action of a sulfuric acid pickling bath containing as a corrosion inhibitor a mixture of ethyletnfi thioglycol and his beta hydroxy ethyl thioe er.

5. A ferrous metal pickling bath comprising sulfuric acid and ethylene thioglycol as a corrosion inhibitor.

6. A ferrous metal pickling bath comprising sulfuric acid and propylene thioglycol as a corrosion inhibitor.

7. A ferrous metal pickling bath comprising sulfuric acid and an alkylene thioglycol having between 2 and 5 carbon atoms as a corrosion inhibitor.

8. A ferrous metal pickling bath comprising sulfuric acid and a mixture of ethylene thioglycol and his beta hydron ethyl thioether as a corro- 0 sion inhibitor. 

